Such systems, application apparatuses and such a sensor unit for measuring a mass flow rate in a fluid, such as a hot-melt adhesive, are known in the prior art. Such a sensor unit uses the principle of convection anemometry and requires no direct interference with the mass flow by moving mechanical parts. The measuring principle requires the heating of a temperature measurement device beyond the temperature of the hot-melt adhesive. The temperature of the hot-melt adhesive is determined by means of a further temperature measurement device. Depending on the flow rate of the hot-melt adhesive surrounding the heated temperature measurement device, a larger or lesser amount of heat is released to the fluid flowing around. Using a control unit, the temperature of the heated temperature measurement device is controlled to be a fixed positive value above the temperature of the surrounding hot-melt adhesive. The required heating power and the associated energy consumption are a measure of the flow rate and thus the mass flow of the fluid. An illustration of the measuring principle can be found in the following publication: Schönteich, B., Fischer, F.: Precise mass flow measurement in highly viscous adhesives—virtually without pressure loss [Präzise Massestrommessung in hochviskosen Klebstoffen—nahezu ohne Druckverluste], in: ifs report of the Institute for Joining and Welding Technology of the Technical University of Braunschweig, edition January 2013, pp. 10-13.
The two temperature measurement devices are arranged according to the prior art with a 90° offset along an axis through a flow channel. Furthermore, the sensors are designed as rod-shaped elements.
A disadvantage of the described prior art has the effect that, due to their area, the sensors generate an additional flow resistance in the mass flow, which adversely affects the pump power required to generate a certain amount of hot-melt adhesive and leads to pressure losses within the system. Moreover, the positioning of the sensors creates turbulences in the hot-melt adhesive which can result in uneven fluid exit particularly in the region near the dispensing nozzle, which adversely affects the application quality of hot-melt adhesive onto a substrate or workpiece. In addition, due to the turbulences, accuracy restrictions with respect to the obtained measurement results are possible. Furthermore, the sensors are not easy to maintain as a unit and not easily removable from the housing.